diff --git a/c_src/commit.c b/c_src/commit.c
new file mode 100644
index 0000000..0b36256
--- /dev/null
+++ b/c_src/commit.c
@@ -0,0 +1,24 @@
+#include "geef.h"
+#include "repository.h"
+#include "object.h"
+#include "oid.h"
+#include <string.h>
+#include <git2.h>
+
+ERL_NIF_TERM
+geef_commit_tree_id(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[])
+{
+	const git_oid *id;
+	geef_object *obj;
+	ErlNifBinary bin;
+
+	if (!enif_get_resource(env, argv[0], geef_object_type, (void **) &obj))
+		return enif_make_badarg(env);
+
+	id = git_commit_tree_id((git_commit *) obj->obj);
+
+	if (geef_oid_bin(&bin, id) < 0)
+		return atoms.error;
+
+	return enif_make_binary(env, &bin);
+}
diff --git a/c_src/commit.h b/c_src/commit.h
new file mode 100644
index 0000000..2878357
--- /dev/null
+++ b/c_src/commit.h
@@ -0,0 +1,6 @@
+#ifndef GEEF_COMMIT_H
+#define GEEF_COMMIT_H
+
+ERL_NIF_TERM geef_commit_tree_id(ErlNifEnv *env, int argc, const ERL_NIF_TERM argv[]);
+
+#endif
diff --git a/c_src/geef.c b/c_src/geef.c
index 59cbe57..84bb429 100644
--- a/c_src/geef.c
+++ b/c_src/geef.c
@@ -3,6 +3,7 @@
 #include "reference.h"
 #include "oid.h"
 #include "object.h"
+#include "commit.h"
 #include "geef.h"
 #include <stdio.h>
 #include <string.h>
@@ -97,6 +98,7 @@ static ErlNifFunc geef_funcs[] =
 	{"oid_fmt", 1, geef_oid_fmt},
 	{"oid_parse", 1, geef_oid_parse},
 	{"object_lookup", 2, geef_object_lookup},
+	{"commit_tree_id", 1, geef_commit_tree_id},
 };
 
 ERL_NIF_INIT(geef, geef_funcs, load, NULL, NULL, unload)
diff --git a/src/geef.erl b/src/geef.erl
index f0bbeb5..54ee2f6 100644
--- a/src/geef.erl
+++ b/src/geef.erl
@@ -11,7 +11,7 @@
 -export([oid_fmt/1, oid_parse/1]).
 
 % objects
--export([object_lookup/2]).
+-export([object_lookup/2, commit_tree_id/1]).
 
 -on_load(load_enif/0).
 
@@ -74,6 +74,10 @@ oid_parse(_Sha) ->
 object_lookup(_Repo, _Oid) ->
     nif_error(?LINE).
 
+-spec commit_tree_id(term) -> binary().
+commit_tree_id(_Handle) ->
+    nif_error(?LINE).
+
 nif_error(Line) ->
     erlang:nif_error({nif_not_loaded,module,?MODULE,line,Line}).
 
diff --git a/src/geef_commit.erl b/src/geef_commit.erl
new file mode 100644
index 0000000..7c54596
--- /dev/null
+++ b/src/geef_commit.erl
@@ -0,0 +1,9 @@
+-module(geef_commit).
+-export([tree_id/1]).
+
+-include("geef_records.hrl").
+
+-spec tree_id(object()) -> oid().
+tree_id(#object{type=commit,handle=Handle}) ->
+    Oid = geef:commit_tree_id(Handle),
+    #oid{oid=Oid}.
diff --git a/src/geef_object.erl b/src/geef_object.erl
index 5fe4a0e..68b6da2 100644
--- a/src/geef_object.erl
+++ b/src/geef_object.erl
@@ -8,7 +8,7 @@
 lookup(#repo{handle=Repo}, #oid{oid=Oid}) ->
     case geef:object_lookup(Repo, Oid) of
 	{ok, Type, Handle} ->
-	    #object{type=Type, handle=Handle};
+	    {ok, #object{type=Type, handle=Handle}};
 	Other ->
 	    Other
     end.